This paper focuses on a downlink beamforming problem in the cognitive radio (CR) communication system where primary users (PUs) coexist with secondary users (SUs). It is assumed that the SU transmitter has multiple antennas and transmits data to another SU's single-antenna receiver by employing the beamforming. By properly designing a beamforming vector, a SU can maximize its channel gain while maintaining an interference with PU below a predefined level. Designing a beamforming vector includes steering beamforming vector direction as well as adjusting transmit power. In order to construct the optimal beamforming vector, the SU transmitter requires knowledge of the downlink channel information of both PU and SU receivers sent on the feedback channel. However, it is impractical to assume that the transmitter has the perfect channel state information (CSI) considering the tremendous feedback overhead. One way to tackle this feedback overhead problem is to use a finite number of feedback bits indicating the index of a predefined codeword in the codebook. We analyze interference to the PU receiver and propose an algorithm to design a beamforming vector considering an error attributed to the partial CSI. The simulation shows that the analysis is quite accurate and that the beamforming vector, designed according to the proposed algorithm, keeps interference to PU below a predefined level while taking into account the error owing to the partial CSI. In addition, we propose a feedback bit allocation mechanism, in order to maximize the gain of the SU link. By using the proposed scheme, the CR system become robust against errors attributed to the partial CSI.

Recently, a cooperative cognitive radio network (CCRN) and multiple-input multiple-output (MIMO) cognitive radio (CR) network have been proposed and investigated individually as promising paradigms for CR networks. Herein, we consider applying the MIMO techniques to a CCRN in order to achieve increased spectral efficiency. Assuming that the system comprises a single-input single-output (SISO) primary user (PU) pair and a multi-input single-output (MISO) secondary user (SU) pair, we propose jointly optimizing the beamforming vector and power allocation for the SU transmitter in order to maximize the rate for the SU while meeting the rate requirement for the PU. As the problem is nontrivial, we circumvent it using the uplink and downlink duality and equality of the left and right eigenvalues. Further, we introduce an iterative algorithm with a proof of convergence. From the numerical results, the proposed algorithm converges in a small number of iterations and outperforms the zero-forcing (ZF) beamforming algorithm while providing an upper bound for the CCRN-MIMO performance. Furthermore, the application of the proposed algorithm is not limited to CCRN-MIMO; it can also be extended to the conventional relay MIMO and/or MU-MIMO with a quality-of-service (QoS) requirement.

Precision instrumentation systems, such as optical receivers and other current-output measurement systems, often contain a transimpedance amplifier (TIA). The commonly used resistive feedback TIA (RF-TIA) has a drawback in its practical implementation; the system bandwidth is limited by the feedback resistor and its parasitic capacitance in high-speed applications. To overcome this drawback, a capacitive feedback TIA (CF-TIA) can be used. The gain and bandwidth performance of the CF-TIA are theoretically equivalent to those of the RF-TIA. Although CF-TIA has been introduced in previous studies for reducing thermal noise and addressing the difficulty in integrating a high resistance with CMOS technology, past analyses have not been in sufficient depth. The ultimate goal of this study is providing designers in this field with helpful design rules and analytical tools. In particular, a transfer function model, the critical design parameters, and a stability analysis are presented. Furthermore, a new CF-TIA configuration involving a simplified integrator DC feedback loop is introduced, which can be deployed as a single stage rather than the conventional two-stage topology. While the applications considered in this study were high-speed sensor measurements and devices with a high input capacitance, such as a laser position detector, the analytical results obtained herein also applicable to a variety of other applications such as emerging biosensors and optical communication system.

In this paper we focus on a SC-FDMA system's resource allocation problem considering the uplink of the 3GPP Long Term Evolution system. We first define a utility function at each sector aiming at maximizing sum of average SINR, and implement the fairness factor which enables the resource allocator to schedule resources fairly among the users. Then, for the fairness-aware utility function, the optimization problem is solved using the iterative Hungarian. In multi-cell system, as the users suffer a uncorrelated inter-cell interference, it is difficult to find the globally optimal radio resource allocation. We propose a distributed allocation method which avoids the drastic interference level changes. Results exhibit that the proposed allocation method has a robustness to the interference variation and can be close to the globally optimal allocation.

As a potential solution for the backhaul of next-generation mobile or low Earth orbit (LEO) satellites, free-space optical (FSO), has received much attention recently. Various techniques have been suggested for employing an intelligent reflecting surface (IRS) in FSO systems, such as anomalous reflection, power amplification, and beam splitting. It is also possible to deliver more power to the receiver (Rx) by collimating or focusing the reflected beam at the Rx lens. In this paper, we propose a phase-shift design of an IRS for beam focusing. In addition, we propose a new pointing error model and an outage performance analysis for the case when the beam width is comparable to or less than the aperture size of the Rx. The analytical results are validated by Monte Carlo simulations. This paper provides essential preliminary results for future research that assumes a focused beam in FSO systems.